THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian date: 2458742.5
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THE DAILY ASTRONOMER
Monday, September 16, 2019 
The Sun's Lost Siblings

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Our Sun did not form alone.   Four and a half billion years ago, when Sol developed from a gaseous nebula, it formed with many other stars that would ultimately become members of an open or galactic cluster.  This cluster traveled along a circuit within the Milky Way Galaxy, following the arc that its primordial cloud described.  Eventually, within less than a billion years of its formation, the cluster dispersed as its component stars moved slowly, but inexorably, away from its center and along individual trajectories through the spiral arms. As the Sun and its lost siblings have turned around the galaxy more than 20 times since their formation, the cluster components are now scattered far and wide through the expansive Milky Way Galaxy.  The hope of many astronomers is to identify these lost siblings.  

Considering how scattered our stellar sisters have become, would it be at all possible to find them?   Well, astronomers have found one already:  HD 162826, a star within the constellation Hercules.   Although it is relatively close at a distance of 111 light years, HD 162826 is slightly fainter than the dimmest stars visible to the unaided eye.    

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The Sun's first identified sibling:  HD 162826     Located within the constellation Hercles, HD 162826 was the first star positively identified as being one of the Sun's siblings, defined as the stars that formed with the Sun about 4.5 billion years ago.   It is possible that the Sun might have more than a thousand "siblings" at different locations within the Milky Way Galaxy.  At a distance of 111 light years, HD 162826 is likely the closest sibling.  


Although astronomers have found one, the Sun's siblings might well number in the hundreds or even more than a thousand.  How can those lost sisters ever be located?   We know that in 2014 an astronomical research team from the University of Texas at Austin determined that HD 162826 was a sibling based both on its chemical composition and by tracing back its orbital trajectory as it moved around the galaxy. A gaseous cloud from which the Sun and its siblings arose had a consistent chemical composition so that each star that formed within it would be chemically similar.    Astronomers can ascertain this chemical similarity through spectral analysis of these stars.   In particular, they look for relative abundance of rare elements such as Barium.   It is likely that propelled remnants from a supernova (explosion of a super massive star) both enriched the Sun's birth cloud and induced the gradual collapse precipitating the star formation.   These remnants would contain a set amount of the heavier elements which the "metal-poor" cloud would have lacked prior the supernova particles' arrival.     As a consequence, stars born out of a specific nebula would be stamped with a particular spectral signature that would, in theory, distinguish them from non-sibling stars.

The Galactic Archeology with Hermes (GALAH) survey aims to observe more than one million stars so as to determine their individual "DNA profile."  The aim, in part,  is to properly identify some of Sol's siblings:  to know where the Sun's sisters have strayed.  These observations will also lend astronomers some insights into the motions within the galaxy, itself:  as leaves in a wind give us information about the direction and velocity of air streams.      We don't know how many of the Sun's lost siblings we'll find through these observations.    We do know, however, that they lurk out there somewhere..



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